1. Field of the Invention
The present invention relates to a dual-polarized antenna and an array antenna, more particularly, to a mobile communication base station antenna for realizing a Space Division Multiple Access (SDMA).
2. Related Art
In general, conventional mobile communication base station antenna has a sharp vertical plane directivity as shown in FIG. 7, so as to suppress interference to other cells. Referring to FIG. 7, in a mobile communication base station antenna 420, a main beam direction 422 of the antenna 420 provides a tilt angle 421 in the vertical plane with respect to a horizontal direction.
In the mobile communications, particularly, in portable phone communications, MIMO (Multiple Input Multiple Output) communication becomes popular. In the MIMO communication, data transmission efficiency can be enhanced by employing plural antenna as transmitting antenna and receiving antenna, respectively. In comparison with communication speed in the case of using one transmitting antenna and one receiving antenna, communication speed in the case of using two transmitting antennas and two receiving antennas is theoretically double, and communication speed in the case of using four transmitting antennas and four receiving antennas is theoretically four times.
In the MIMO communication, correlation of signals from respective transmitting antennas to respective receiving antennas becomes important. In particular, a channel capacity of the transmitting antenna is influenced by a correlation coefficient between the respective transmitting antennas, and a channel capacity of the receiving antenna is influenced by a correlation coefficient between the respective receiving antennas. For example, in the 4×4 MIMO communication using four transmitting antennas and four receiving, antennas, when there is “no correlation”, namely, the correlation coefficient is substantially zero (0), between the respective antennas, the communication speed is close to 4 times which is theoretically established. On the other hand, when the correlation coefficient is substantially 1, the effect of the MIMO communication cannot be expected. In practical use, it is preferable that the correlation coefficient between the antennas is 0.7 or less.
For example, Japanese Patent Laid-Open No. 2005-203841 (JP-A 2005-203841) discloses a conventional polarization diversity antenna element used in a mobile phone base station antenna.
So as to decrease the correlation coefficient, it is sufficient to spatially or electrically divide (separate) the antenna. By way of example only, the conventional polarization diversity antenna element used in the mobile phone base station antenna disclosed by JP-A 2005-203841 is a two-system antenna which is divided by polarization. Therefore, if such an antenna is used for an array antenna, it can be converted into a base station antenna for 2×2 MIMO communication.
By way of example only, for the case of 4×4 MIMO communication, if a distance between two antennas is increased, namely, the two antennas are distant from each other, the correlation coefficient will be decreased in accordance with the increase in distance. Therefore, referring to FIGS. 8 and 9, it is requested that a distance between two polarization diversity array antennas 505, 505 that are juxtaposed (FIG. 8) or vertically arranged in a column (FIG. 9) should be increased as much as possible. However, there is another request inconsistent with the former request, namely, it is also requested that the distance between the two polarization diversity array antennas 505, 505 should be decreased as much as possible, since a volume (space) required for antenna installation increases when the distance between the two antennas is increased too much.